The present disclosure relates, in general, to image sensors and, in particular, to charge-domain analog readout circuits for such sensors.
Image sensors find applications in a wide variety of fields, including machine vision, robotics, guidance and navigation, automotive applications, and consumer products. In many smart image sensors, it is desirable to integrate on-chip circuitry to control the image sensor and to perform signal and image processing on the output image. Unfortunately, charge-coupled device (CCD), which have been one of the dominant technologies used for image sensors, do not easily lend themselves to large scale signal processing and are not easily integrated with CMOS circuits. Moreover, a CCD is read out by sequentially transferring the signal charge through the semiconductor, and the readout rate is limited by the need for nearly perfect charge transfer.
Active pixel sensors (APS), which have one or more active transistors within the pixel unit cell, can be made compatible with CMOS technologies and promise higher readout rates compared to passive pixel sensors. Active pixel sensors are often arranged as arrays of elements, which can be read out, for example, a column at a time. Each column can be read out at one time, driven and buffered for sensing by a readout circuit.
An exemplary voltage mode circuit is shown in FIG. 1. Each column, such as the column 10, includes a source-follower 12. The column 10 is enabled by a switch 14 to drive its output onto a common bus line 18. Other columns such as 16 can alternately be driven onto the bus line 18. The bus line includes an inherent stray capacitance shown as 20. Typically, a single constant current 22 is used in common for all the source-followers.
The source-follower 12 is formed with a drain, source, and gate. The speed of such a source-follower can be increased by increasing the channel length which also requires increasing the current on the source 22. However, increasing the channel length has the undesirable effect of increasing the stray capacitance 20, thereby decreasing the speed. Such a trade-off results in a little improvement in speed because the increase in capacitance tends to offset the increase in power.
Accordingly, it is desirable to provide a circuit which has improved capacity for reading out signals from an array of active pixel sensors.